Gelation coating process



June 6, 1944.

I GELATION COATING PROCESS Original Filed Deg. 19, 1940 F/e. /4 0 l5 CHARLEsR l-movcs c. R. FORDYCE ETVAL 2,350,743

Patented June 6, 1944 GELATION COATING PROCESS Charles B. Fordyce and-Gerard J. Clarke, Rochester, N. Y., asslgnors to Eastman Kodak Comter, N. Y., a corporation of New Original application December 19, 1940, Serial No.

. be! 8, 1943, Serial No. 513,415

This invention relates to coating, and more particularly to a process for coating objects by movin them in various cellulose ester lacquers which have the ability to solidify by gelation as distinguished from ordinary cellulose ester-lacquers.

This application is a division of our eopending application, Ser. No. 370,853, filed December 19, 1940.

.A is well known, cellulose ester coatings may be applied to the surface of various objects made of wood, metal and other substances by dipping them in certain cellulose ester lacquer compositions, or by brushing or spraying such compositions onto the object. The dipping process involves the immersion of the article to be coated in the lacquer bath and gradual withdrawal therefrom with the object of producing a uniformly even coating. The success of the operation depends upon proper control of uniformity of coating, both as to thickness and surface appearance. While the lacquer dipping process presents many desirable features for the economic application of surface coatings, especially on small articles, there are a 'number of outstanding difilculties which severely limit its use. For example, upon withdrawal of the object from the lacquer solution there is a pronounced tendency for the uncured coating to 11m, building up ridges of uneven thickness upon drying. The coating of articles of irregular shape presents special diiilculty because of creeping of the fresh coating away from edges or comers, leaving a very thin coating at those places. In order to I avoid these difliculties, it is usually necessary to apply only a very thin coating to the object so that such unevenness is less pronounced. However, under such circumstances, it is difllcult to build up a heavier coverage by repeating dipping because the solvents in the dipping bath attack the previously applied coating, causing it to become even more uneven and, in some cases, actually to remove it completely on various localized areas.

Various attempts have been made to overcome the abovementioned difliculties which are comeffected. Solvent mixtures of alcohols and hy-- drocarbons have been suggested for this purpose but such an expedient has never proved satisfac and this application Decem- 1 Claim. (Cl. 117-103) tory, or even commercially operative, for the reason that such solutions undergo such a severe or sudden solidification upon cooling that the cellulose ester material precipitates from the solvent giving white or opaque coatings which, upon curing, areextremely brittle and thus entirely unsatisfactory. Such coatings have such a pro-' nounced tendency toward separation of the liquid from the solid material as to give an effect which is known as watering, rather than undergoing homogeneous evaporation as is the case with customary lacquer solvents.

A further attempt to eliminate some of the above-mentioned difficulties encountered in the use of dipping lacquers is based upon the use of cellulose nitrate lacquers and the chilling of the dipped object in order to arrest fluidity of the .coatingto such a degree that running of the composition is less pronounced than with a lacquer employed at ordinary temperatures. This process has the disadvantages that such refrigeration is expensive and that only certain cellulose derivatives, specifically cellulose nitrates, can be made to give satisfactory commercial operation.

This invention has as an object to provide an entirely new and improved method of producing cellulose ester coating on the surface-of various articles which can be immersed in a bath of dipping lacquer. A further object is to provide a rapid and reliable method ofproducing uniform, cellulose ester coatings on the surface of such objects. A still further object is to provide a process of producing cellulose ester lacquer coatings which will not run, become uneven, or will not lift off or develop thin spots upon the application of successive lacquer layers. Another object is to provide a. method of producing surface coatings ona. wide variety of plain and irregularly surfaced objects not hitherto susceptible of coating by-ordinary lacquer technique. Other objects Will appear hereinafter.

These objects are accomplished by' the following invention which, in its broader aspects, involves dipping the object to be coated in a solution of a cellulose organic acid ester dissolved in a liquid which is a solvent for the ester in question only at temperatures above 50 C. and which at a temperature between 10 and 50 C. will form a self-supporting gel, said solution being maintained at a temperature above 50 C., removing the dipped article from the solution and thereaftercooling the clipped article to a temperature between 10 and 50 C., followed by curing of the residual solvent therefrom.

In the following examples and descriptlmwe;

have set forth several'of the preferred embodiments of our invention, merely for purposes of illustration and not as a limitation thereof.

In the accompanying drawing,' we have illustrated one type of apparatus suitable for carrybath of dipping lacquer 2. Any suitable heating fluid, such as hot water, steam or the like, may be supplied to jacket 3 of the tank through inlet conduit 4, the fluid flow through which is controlled by valve 5. The heating fluid passes out of the apparatus through valve 6 and conduit 1, as shown. In order to prevent heat losses the outside of the tank may be provided with suitable lag 8.

Numeral '8 designates a second dippin tank which, under ordinary circumstances, is not lagged. This tank contains a body of non-solvent liquid which is supplied thereto by means of inlet conduit ll controlled by valve l I. Liquid is removed from the tank through valve l2 and outlet conduit it. Both tanks may be q pped with constant level devices (not shown) for maintaining any desired level of liquidtherein. In the case of tank 9, the desired level may alternatively be maintained by an appropriate adjustment of valves II and i2.

Positioned adjacent to both tanks is a lowering and elevating mechanism It comprising standard I! which is bored to accommodate shaft It which carries head ll. Provision is made for raising or lowering the head on shaft Ii through collar It provided with adjustable screw member i9 threaded through the collar and adapted to bear against the shaft thereby fixing the position of head I! which rests thereon. Mechanism i4 is also provided with a windlass 20 the shaft 21 of which is rotatably mounted in suitable bearings in lugs 22 integral with standard It adjacent its upper part. Shaft 2i is rotated by means of crank 23.

A steel cable or cord 24 is attached to the shaft of. the windlass as shown. and passes over pulleys 25 and 26 rotatably mounted in the chan-' nel 21 as shown. The outer end of cord 24 is rove through eyelet 28 in dipping rack 29. This rack has secured thereto a plurality of hooks 30 to which the objects iii to be dipped are temporarily attached by any convenient means.

The operation of, the apparatus will be apparent from a consideration of Figs. 1 and 2. Dipping rack '29 is raised to an appropriate height by winding up cable 24 by means of Windlass 20. The objects to be dipped are then attached to the hooks to, head i1 is then swung 'over until rack 29 is directly over the dippin bath 2, whereupon the windlass is operated in the reverse direction. thus lowering the objects slowly into the body of dipping lacquer where they are allowed to stand until free from surface bubbles. The Windlass is then operated slowly so as to evenly and gradually withdraw the objects from the lacquer at a uniform rate, time being allowed for the objects to acquire on their surfaces a layer of the lacquer of appropriate thickbut they are included- The tank. as shown, should preferably be so constructed as to protect the atmosphere above the surface of the dope from air currents and to provide a fairly high concentration of solvent vapors in that vicinity. While no hard and fast rule can be laid down, it may be said that after remaining in this atmosphere at 40 C. for about 30 seconds the coating will be set under the influence of .the lower atmospheric temperature prevailing at that point to the point where fingerprints or. other contacts will not cause damage, notwithstanding that the coating retains practically all of the original volatile solvent at this stage of the operation.

Following this gelation stage. rack 29 may then be wound up until all of the objects can clear the top of the tank. Head i1 is then swung on 'shaft I as a pivot and the lacquer coated objects left to cure by evaporation of solvent, which may be carried out at room temperatur or in a moderately warm atmosphere.

As to the actual gelation step, it should be pointed out that if a dope is employed which gels at ordinary room temperatures, no positive cooling of the atmosphere in which gelation takes place is necessary. However, if a dope is used which gels only belowroom temperatures special provision must be made for refrigerating the atmosphere in the immediate vicinity of the dipped objects. Alternatively, .the objects may be dipped in a bath of cold non-solvent liquid such as toluene. In the latter case the head I] is swung over until the rack is in a position to be lowered into the'bath contained in tank 9. This tank may, for example, contain a body of'cold toluene maintained at any desired temperature by cooling coils or other appropriate device. Rack 29 is then lowered as in the pre- 'lects in a non-solvent liquid is also employed when it is desired to deposit very heavy coatings, it having been found that when a heavy coating is deposited and solidified by gelation as.

previously described, if the gelled coating is treated with a non-solvent liquid such as toluene, for example, the residual solvent may be removed from the coating much more evenly and uniformly than is the case where no non-solvent treatment is employed. This is presumably because of the extracting effect of the non-solvent liquid on the solvents contained in the gelled material, or the replacement of the original solvent in the gelled structure by a more volatile and therefore .more easily removable solvent than the original liquid. I

As to the non-solvent liquid itself, this may be any liquid which is non-solvent with respect to the cellulose ester in question. However, we prefer to emplo'y non-solvents such as toluene, carbon tetraici'iloride;-xylene, benzene and ligroin. 0f liquids. locus is probably the most satisfactoni-f alliifound operation. It may n-solvent liquid is prefscan the basis of its power of dissolving or Hitting the solvent from the gelled compositiongsince this facilitates removal of solvent-from the deposited coating and leaves less residual solvent to be removed in the air curing stage.

erably to beise'leh shrinkage of the deposited coating, which is considerable, takes place' uniformly throughout the coating layer, and thus precludes the possibility of buckling or distortion of the surface. This treatment'may be unnecessary in the case of very thin coatings, but is desirable with heavier layers In either case the deposited wq er coating shrinks down tightly and evenly around the object, producing a highly satisfactory, smooth The general nature of the solutions which may be employed in the laoquering process of our invention have been referred to above. These solutions are what may be called gel dopes or lacquers, that is, cellulose ester solutions which are fluid above 50 C. but at a temperature within the range of 10-50 C. set to a non-fluid self-supporting gel. The characteristics of such compositions are more particularly set forth in the U. S. patents to Fordyce and Hunter 2,319,051, 2,319,052, 2,319,053, 2,319,054 and the U. 8'. patent to Fordyce and Gould 2,319,055. The instant application relates specifically to those compositions in which a gellable cellulose organic acid ester of the type referred to and more particularly illustrated by the specific examples to follow, is dissolved in a solvent mixture composed of a alkylene dichloride of from 2-4 carbon atoms and lulose cater to be applied over a previously formed article of the same composition.

' Example 2 To a solution of 100 parts of cellulose acetate of 40.5% acetyl content dissolved at 70C. in 500 parts of a solvent mixture of 65% propylene dichloride and 35% tertiary butyl alcohol was added 2 parts of finely divided carbon black and. stirred,'.filtered, and

the resulting mixture well held in a dipping tank at 50. C. A variety of kitchen utensils were dipped in the solution and gelled at-30-35 C. to provide on the'handles a uniform black coating, thickness of which after curing was found to be .01". The coatings were much heavier than could be obtained with black paint, and were outstanding in resistance to hot I water and to mechanical abrasion.

an aliphatic alcohol of from 2-4 carbon atoms.

The cellulose esters which have been found most suitable are cellulose acetates of 38-42% acetyl and cellulose acetate propionates or cellulose acetate butyrates of at least 43% total acyl content and containing not over 40% of the higher acyl radical. v

The concentration of the lacquer solutions may vary depending upon the'thickness of the coating desired, but will usually fall within the limits of 25-20% of the ester, based on the weight of the total composition. Plasticizers may be employed, if desired, up to an amount representing 50% of the cellulose ester. However, high plasticizer contents are to be avoided because of a tendency toward interference with the gelling properties of the composition. On the other hand oneofthe outstanding advantages of the compomtions of our invention is that one may deposit very heavy cc'iiti'ngs without the use of any plasticizer whatever," as compared with molding operations usuallyemployed to deposit such thick coatings which require considerable quantities of plasticizer in order to obtain a suitable flow under pressure. 7

Our invention will be more fully understood by reference to the following typical examples of our process. I

Example 1 For application of very thin clear coatings a solution was prepared by dissolving 5 parts of a high viscosity cellulose acetate of 39% acetyl content in 65 parts of propylene dichloride and parts of tertiary butyl alcohol at 60 C. The solution was then placed in a dipping tank at 45 C. A molded comb of cellulose acetate plastic was dipped into the solution and withdrawn at a rate adjusted to prevent running and wrinkling of the coating and the latter caused/to set at a temperature of 30-35 C. It was then cured at 50 C. Other pieces of cellulose acetate plastic were also dipped, and the products were uniformly brilliant in appearance and free from wrinkling effects which would result if solvent 7 A block of wood was machined into an oblong piece 5" wide, 6" long, and 1'-' thick with beveled edges on the upper surface. It was dipped in the lacquer to provide a-v'ery smooth surface, then dipped again in the black pigmentedlacquer, the coating gelled and cured as previously described. It was then carefullysanded and finally dipped into a clear lacquer of identical composition to that used for the first two coatings excepting that it contained no pigment. After thorough curing,the coated block presented the appearance of a molded plastic.

One of the outstanding features of our process is the fact that objectsof a variety of shapes may be clipped and removed from the dipping bath without running of the lacquer, as is the case with prior art lacquers which have a pronounced tendency to build up ridges of accumulated material or to break away from sharp edges as previously described. Another feature of.our invention is the fact that the solvents employed in these gel dope lacquers are of such a nature thatsuccessive coatings may be applied one after another without thinning or spotting out of the lacquer previously deposited. This tion by gelation and; curing, thelacquered surface may be rubbed down .with emery cloth, sandpaper, or other abrasive and successive pigmented layers deposited. A final layer of clear lacquer may then be deposited, to produce a high gloss finish if desired.

It goes without saying that almost any form or shape of object may be lacquered by our improved dipping process. In fact, it is one of the features of our invention that, due to the fact of our lacquers solidifying or setting to a gel immediately ,upon reaching the critical temperature range of the solution in question, no appreciable running or accretions of cellulose ester materials occurs. Furthermore, since these coatings shrink down tightly around the objects to which they 'sreepplied,sneveneostinzisdepositedoverthe tion temperature in"s, .polvent mixture composedofsnslkylenedichlorideoiiromfl-iesrboo stoma and an sliphstie mum of from 2-4 carbon stoma, coating the srticle with the solu- 5 tlon above its zelstion temperature. Iellinl the coating, and removin: the residue! solvent.

CHAR-LIB R. I'QRDYCI. GERARD J. CLARKE. 

